A hard disk drive (HDD) includes at least one platter that is coated with a magnetic medium. The platter is axially mounted to a rotating spindle. An arm pivots parallel to the coated surface of the platter and carries a read/write head. A servo motor, such as a voice coil motor, moves the arm to position the read/write head over a selected one of several concentric tracks recorded on the magnetic medium.
A servo control system drives the servo motor and receives a position feedback signal from the read/write head. The feedback signal is generated when servo codes, which are interspersed with data recorded in the concentric tracks, pass under the head. The servo control system uses the feedback signal to periodically adjust the position of the arm so that the read/write head aligns with the selected track.
The servo codes are recorded on the magnetic medium when the disk drive is manufactured. The servo codes can be positioned at selected intersections of the concentric tracks and imaginary line segments that radiate outward from the spindle. The resulting pattern of servo codes can appear as pie slices and can be referred to as servo wedges. The frequency of the feedback signal is based on the rotational speed of the spindle and the number of servo wedges. Increasing the frequency or servo sampling rate of the feedback signal can improve position tracking between the head and the selected one of the concentric tracks. The servo control system therefore becomes more robust against disturbances, e.g. vibration and shock, when the servo sampling rate increases.
In some HDDs, such as those commonly used in desktop computers, the platter diameter is about 3.5 inches and the spindle speed is about 7200 revolutions per minute (RPM). These HDDs can provide a servo sampling rate as high as 50-60 kHz (15-20 μS servo period). In other HDDs, such as small form-factor HDDs, the platter diameter can be less than about 1.8 inches. Small form-factor HDDs are commonly used in portable and handheld mobile applications. To meet low power dissipation requirements, small form-factor HDDs generally have spindle speeds between about 3600 and 4200 RPM. Small form-factor HDDs also have between about 120 and 180 servo wedges.
Based on the aforementioned attributes, small form-factor HDDs typically provide a servo sampling rate between about 8.3 and 12.5 kHz (80-120 μS servo period). With this low servo sampling rate, any disturbance that occurs while the head is between servo codes and moves the head by more than about 10% of the track width can become problematic for drive performance and read reliability. Some prior-art servo control systems inhibit reading and writing until the head realigns with the selected concentric track. Such a solution can have undesirable effects in high data rate, high vibration applications such as hand-held music and/or video players.